Revert "io: detect Writers that access io.Copy's buffer after returning"

This reverts CL 466865.

Reason for revert: Causing trybot flakiness due to net/http race,
roll back until net/http is fixed.

For golang#58168

Change-Id: I3129deb996abe6466eccf933fe93cbbaf72ae217
Reviewed-on: https://go-review.googlesource.com/c/go/+/467895
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Damien Neil <dneil@google.com>

src/io/io.go

@@ -14,7 +14,6 @@ package io
 
 import (
 	"errors"
-	"internal/race"
 	"sync"
 )
 
@@ -428,14 +427,6 @@ func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
 		nr, er := src.Read(buf)
 		if nr > 0 {
 			nw, ew := dst.Write(buf[0:nr])
-			if race.Enabled {
-				// Write over the buffer to help the race detector
-				// detect Write calls that access the buffer after returning.
-				// Use something other than zeros to make errors more obvious.
-				for i := range buf[:nr] {
-					buf[i] = 0xdd
-				}
-			}
 			if nw < 0 || nr < nw {
 				nw = 0
 				if ew == nil {